When the factors that affect intelligence are debated, heredity and environment come up immediately. Factors that affect intelligence are either heredity or environment, and, except for spiritual or supernatural intervention, those are the only available factors.
Many people say there is anecdotal evidence that other factors influence intelligence, but actually, “anecdotal” is very close to being a synonym for “unreliable.” That is not the way the word is used in everyday speech, but when scientists speak anecdotal often either means “in itself unproven,” or “leading to a false conclusion.”
For example, Mary Jones watched a lot of the Public Broadcasting System when she was pregnant, and her child received very high scores on IQ tests. There is no real link between maternal TV watching and intelligence. The evidence that the PBS show Front Line raises children’s IQ is anecdotal.
Ignoring the anecdotal, here is a consideration of factors that have been shown to affect intelligence.
Heredity, loosely, is the sum of genetic instructions a child receives from his or her ancestors. However, not every gene carries a clear and determining command and not every genetic instruction will be followed.
The human genome contains about 23,000 gene pairs. Some clash, some counter one another, and some potentiate. To potentiate means to increase the effect of other genes, to magnificent or devastating effect.
Some genes have effects that are helpful, even lifesaving, in one context but are deleterious in another. The most famous example of this dichotomy is probably the gene for sickle cell disease. Sickle cell trait can be a lifesaver in some environments, but sickle cell disease can kill. There are almost certainly genes with analogous effects on intelligence, that are sometimes helpful, and sometimes devastating.
What is certain is that one gene determines intelligence. If there were such a gene, it would have been isolated long ago. Various genes promote intelligence, and some stymie it. Everyone inherits a mixture of genes.
Environment, loosely, is everything that is not heredity. Maternal nutrition, social circumstance, cultural patterns, wealth, poverty, peace and war all affect the development of intelligence. Plants grow best in optimum conditions, though they persevere in hardship, and human intelligence similarly develops better in some circumstances than in others.
That is not a common sense conclusion; it is hard science.
Some conditions that promote intelligence are well-known, and others are mysterious. That being the case, the search for practical knowledge about intelligence branches into two quests. How do I protect and enhance my own intelligence, and what is likely to promote intelligence in the next generation? What follows are generally accepted scientific conclusions about what promotes intelligence and what damages it.
Promoting Personal Intelligence
Exercise, particularly aerobic exercise, maintains intelligence. Humans with good lung function, who continue to exercise throughout life, tend to stay sharper. (Yet look at Stephen Hawking. As he has ALS, he never exercises.)
Nutrition also promotes intelligence. Severe childhood malnutrition has been shown to lower intelligence test scores. To promote life-long intelligence, eat a balanced diet that promotes low cholesterol, low triglycerides, and sufficient stores of iron and other minerals. One simple path to a good diet is to eat the foods grandmother wanted you to eat, not the ones the TV suggests.
Social life promotes intelligence. As social animals, our optimum intellectual development comes with a place in society, whether a club, a job, or a partnership.
Continuing education also promotes and protects intelligence. While it is true that someone with a legitimate PhD is likely to have started out highly intelligent, it is also true that someone who continually educates herself will probably maintain an intellectual edge over someone who does not.
Producing Intelligent Children
Since there are hereditary components to intelligence, seek an intelligent mate. Ethnicity is irrelevant to this search, except for two factors that influence intelligence in contradictory ways. Children born to parents whose genetic inheritances differ widely are likely to be more vigorous than children whose parentage draws from a smaller gene pool, and this vigor may promote intelligence.
On the other hand, children raised within a clearly identified social group may be more grounded, more self-assured, and healthier, other things being equal, than children who grow up with less of a sense of belonging. This increased self-assurance may provide a basis for a high-achieving life.
Potential parents must consider the consequences of their choices when they choose a mate from near or far. On the other hand, since people generally do not choose whom they love intellectually, it makes sense to compensate for these factors in childrearing.
Pregnant women should not drink or smoke, and should immediately seek prenatal health care. They should avoid infections, stress, and exhaustion. As a rule, they should get mild exercise.
After birth, children need peace, security, and a sense of their place in the universe. They need a gentle, thorough, grounding in the ways of the world. They need a chance to learn. Parents cannot always provide these things, but they must try.
Opinions differ about what intelligence really is, and so ideas about what specific factors affect intelligence vary widely, too. Certainly, parents should make thoughtful choices to promote intelligence in their children, and societies should take care of their children. That way, society will have a larger number of intelligent members, who are more likely to be able to make optimum use of their gifts.